UK pipeline
decommissioning provides potential for innovation
Need for new lifting, cutting, and trenching techniques
MickBorwell
Oil & Gas UK
Oil & Gas UK
Since 1966, 45,000
km (27,962 mi) of pipeline has been installed in the North Sea to transport hydrocarbons from the UK
continental shelf (UKCS) to shore. Of this pipeline, less than 2% has been
decommissioned.
The UK government
and industry continue to focus on maximizing recovery of around 15-24 Bboe from the
UKCS, and 2013 brought record investment in new projects. Collaborative work
has resulted in fiscal change and technological advances, but as the basin
continues to mature, decommissioning is emerging as a parallel and growing
business opportunity.
Decommissioning
expertise is available within the UK supply chain, but without significant
activity in this area, the sector has not been fully tested. To help
contractors better understand the opportunities, Oil & Gas UK has produced
several documents.
In its "Decommissioning Insight" published in
2013, the association forecasts that between 2013 and 2022 more than 2,300 km
(1,429 mi) of pipeline, infrastructure from 74 fields, more than 70 subsea
projects, and about 130 installations are scheduled for decommissioning at a
total forecast expenditure of £10.4 billion ($17 billion).
Inventory of UKCS
pipelines
The pipelines
mentioned in the forecast represent a fraction of the extensive network of
pipeline currently installed in the North Sea to transport oil and gas
production to host platforms or to shore. Overall, the UKCS pipeline inventory
covers a broad range of equipment designed to accommodate the transportation of
many different fluids under diverse conditions, varying water depths, and
different oceanographic environments.
In many cases, the
existence of nearby pipeline infrastructure has led directly to the
exploitation of marginal fields that would otherwise be uneconomic. Such
opportunities remain a key factor in the timing of any pipeline
decommissioning. A more detailed description of the different types of pipeline
infrastructure can be found in Oil & Gas UK's 2013 report, "The
Decommissioning of Pipelines in the North Sea Region."
Trunklines represent
the major element of subsea infrastructure transporting large quantities of oil
and gas from offshore to onshore receiving facilities and end users across
Europe. They account for 18% of the total number of pipelines and 63% of the
total pipeline length in the North Sea inventory.
Such pipelines
include some of the longest in the North Sea, often with diameters of more than
30 in., and tend to be installed offshore using the S-lay pipelay method from a
specialist lay vessel.
The pipeline
inventory also includes rigid flowlines, flexible flowlines, umbilicals, and
power cables, as well as associated equipment such as the concrete mattresses
used extensively in the UKCS to provide protection and stability to subsea
pipelines, cables, and umbilicals. These flexible mattresses are typically
manufactured by joining different shapes of concrete blocks together with
polypropylene or Kevlar rope. Oil & Gas UK estimates that 35,000-40,000
mattresses have been deployed since operations began in the North Sea.
While pipelines are
integral to field life extension and future development opportunities, some
fields in the UKCS have reached the end of their economic life. Specific parts
of the pipeline system naturally become redundant, and with no potential future
use, they are available to be decommissioned.
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Seven Navica reeling vessel. (Image reproduced
with permission from Subsea 7)
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Decommissioning
to date
Oil and gas pipeline
decommissioning has been taking place in the North Sea since the early 1990s,
when the Crawford field pipelines were decommissioned. Since then, pipeline
decommissioning has continued at a modest rate and only when all potential
reuse options for the infrastructure, including new field developments, have
been carefully considered.
Less than 2% of the
North Sea pipeline inventory has been decommissioned, and of the pipelines
which have been decommissioned, 80% are less than 16-in. in diameter. Half of
the larger diameter pipelines (16 in. or greater) decommissioned to date were
removed; these were all infield pipelines less than 1 km (0.6 mi) long. The
longest large diameter trunkline to be decommissioned so far is the 35-km
(21.7-mi) Piper A to Claymore 30-in. export line, which was decommissioned in
situ.
Under current
regulations, decommissioning of oil
and gas pipelines is considered on a case-by-case basis using the
comparative assessment (CA) process to determine the best option for
decommissioning. The CA process enables the particular diameter, length, and
configuration of individual pipelines to be taken into account when considering
decommissioning options against the criteria of safety, environmental impact,
cost, and technical feasibility.
Health and safety is
a dominant factor in any CA, with the focus aimed at minimizing the long-term
risks to other users of the sea and the short-term risks to those carrying out
decommissioning operations. An integral part of the process is the
environmental impact assessment, which is prepared to support all pipeline
decommissioning plans.
Each decommissioning
solution needs to be considered on its individual merits, as pipeline
installations vary widely according to model, location, environment, and
maintenance status. It is at the CA stage, when a number of options are
considered, that significant opportunities exist for supply chain companies to
develop innovative technologies for decommissioning pipelines.
Opportunities for
innovation
When evaluating a
preferred option for decommissioning a pipeline and its associated equipment,
the availability and track record of technology used in previous projects
provides the context for the other key CA criteria of safety, environmental
impact, and cost.
Supply chain
companies specializing in particular services will have the opportunity to
develop innovative techniques in the key technology areas for pipeline
decommissioning, many of which are in their infancy. These are:
- Pipeline cleaning
- Trenching, burial, and de-burial
- Subsea cutting
- Lifting
- Reverse installation methods
- Mattress removal.
Pipeline cleaning is
performed prior to decommissioning and involves the depressurization of a
pipeline and the removal of any hydrocarbons in accordance with the Pipelines
Safety Regulations. At this stage there are opportunities for companies skilled
at minimizing the potential contamination of the marine environment.
The technology for
trenching and burial of pipelines during installation is well established, and
a number of contractors offer a range of trenching tools capable of trenching
and burying pipelines of various diameters in all soil types. There is,
however, limited experience of existing pipelines, laid on the seabed surface,
being buried specifically for decommissioning in situ.
While there are
different methods and types of equipment for cutting pipelines subsea using
"cold cutting" tools such as abrasive water jets, diamond wire
cutting, reciprocating cutting, and hydraulic shears, significant opportunities
exist for contractors capable of developing new technologies to improve these
techniques. These might include automated techniques to help reduce the use of
divers in these activities. Lifting sections of infrastructure from the seabed
is another area where innovative thinking is in demand. The "cut and
lift" process of decommissioning requires cut sections of pipeline to be
lifted from the seabed to a transportation vessel; supply chain companies
providing innovative cutting techniques could help increase efficiency in this
area by reducing the duration of lifting operations for long lengths of
pipeline.
Reverse installation
methods encompass both reverse reeling and reverse S-lay techniques. The
process by which rigid or flexible pipelines can be recovered from the seabed
by reeling them from the seabed using a specialist reel vessel is known as
"reverse reeling."
For rigid pipe,
there are a limited number of specialist reel vessels available from the
leading installation contractors. These vessels are usually engaged in
installation activities, but can be adapted to recover pipelines as part of a
decommissioning project. Subsea 7's Seven Navica is one vessel
capable of performing this work.
For larger diameter
and concrete coated trunklines, the industry is considering a reversal of the
S-lay installation process by which pipelines could be removed and recovered on
to the deck of a specialist S-lay vessel. However, this has not been done in
the North Sea, and more study is needed before the technique can be considered
feasible for decommissioning long distance large diameter pipelines.
As yet, no
established technique or technology has been universally adopted for mattress
recovery. Solutions developed by contractors will need to take into account the
age and condition of the mattresses being recovered.
Regional variations
Oil & Gas UK's
2013 "Decommissioning Insight" highlights the contrast between
different UKCS basins, noting that in the central and northern North Sea (CNS
and NNS), decommissioning of pipelines and mattresses is estimated to cost more
than £400 million ($655 million) from 2013 to 2022. Over this period, nearly 40
trunklines (130 km/81 mi), 115 rigid and flexible flowlines (420 km/261 mi), 87
umbilicals (250 km/155 mi), and almost 900 mattresses have been identified for
decommissioning in these basins.
The forecast
indicates significant expenditure will take place from 2019 to 2022, suggesting
that pipeline decommissioning will occur toward the latter end of
decommissioning programs. The peak in 2019 can be attributed to at least 10
pipeline decommissioning projects.
While containing a
similar number of pipelines to the southern North Sea (SNS), the
decommissioning of rigid and flexible flowlines in the CNS and NNS basins is
more expensive, suggesting a greater degree of complexity in these regions.
Over the same period
in the SNS and the Irish Sea, four trunklines (64 km), 116 other pipelines
(1,300 km/808 mi), and 21 umbilicals (150 km/93 mi) will be decommissioned at a
cost of around £100 million ($164 million). Additionally, 2,100 mattresses have
been scheduled for decommissioning.
While these
decommissioning activities represent a fraction of the overall market of oil
and gas activities, they are part of a burgeoning sector. By making more
information on decommissioning available, Oil & Gas UK aims to help the
industry prepare for decommissioning projects, increase the efficiency of
processes involved, and help ensure that future projects are enabled by an
"at the ready" supply chain.
02/05/2014
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